4/26/2021 0 Comments Dc Potentiometer Pdf
This is essential so that once the instrument has been calibrated on x 1 range, calibration of the x 0.1 range is not necessary.The long slide wire is awkward, and even for the length shown cannot be read to a very great degree of precision.
Modern laboratory type potentiometers use calibrated dial resistors and a small circular wire of one or more turns, thereby reducing the size of the instrument. The circuit of a simple DC Cromptons potentiometer theory is shown in the below figure. There is one dial switch with fifteen steps, each having a precision resistor. For the case shown, the resistance of slide wire is 10 and the dial resistors have a value of 10 each. Thus the dial has a total resistance of 150 and in addition, the slide wire has a resistance of 10. The working current of the potentiometer is 10 mA and therefore each step of dial switch corresponds to 0.1 V. The slide wire is provided with 200 scale divisions and since the total resistance of slide wire corresponds to a voltage drop of 0.1 V, each division of slide wire corresponds to 0.1200 0.0005 V. It is quite comfortable to interpolate readings upto 15 of a scale division and therefore with this potentiometer it is possible to estimate the readings upto 0.0001 V. A key and a protective resistance (usually about 10 K) is used in the galvanometer circuit. In order to operate the galvanometer at its maximum sensitivity provision is made to short the protective resistance when near the balance conditions. Supposing the value of emf of the standard cell is 1.0186 V, the dial resistors put at 1.0 V and the slide wire is put at 0.0186 setting. The protective resistance is kept in the circuit in the initial stages so as to protect the galvanometer from being getting damaged. Final adjustments are made for zero deflection with the help of the rheostat. ![]() With the protective resistance in the circuit, the potentiometer is balanced by means of the main dial and the slide wire. The dial settings are kept exactly the same as in the original standardisation process. If the new reading does not agree with the old one, the second measurement of unknown emf must be made. The standardisation should be again checked after the completion of the measurement. This potentiometer is a form of DC Cromptons Potentiometer. ![]() The two ranges are obtained by using two resistors R 1 and R 2 and a range selecting switch S.The operation of the duo range potentiometer may be more easily understood and analysed by redrawing in its simplified form by omitting the galvanometer and calibration (standardising) circuit. The main dial has fifteen steps of 10 each and therefore it has a total resistance of 150. The measuring circuit current I m must be equal to 10 mA in order to produce a voltage drop of 1.6 V across the measuring circuit resistance R m i.e., the resistance of the main dial and the slide wire in series. When the range switch S is thrown to position x 0.1, the measuring current I m must be reduced to 110 of its original value i.e., 1 mA in order to produce a voltage drop of 0.16 V across the measuring circuit resistance R m.
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